What is Spin Welding?

The process of spin welding uses heat
generated by rotational friction at the joint line to weld
thermoplastic parts with rotationally symmetric joints. The
spin welding machine applies pressure axially while rotating
one part against its stationary mate, and the resulting
friction generates heat that melts the parts together.

Material Considerations

Materials that can be friction (i.e. vibration) welded can also
be joined with by spin welding. The semicrystalline thermoplastics are more
readily joined using spin welding than ultrasonics. Using compatible
polymers, spin welding is capable of making reliable hermetic seals.
Far-field welding is easier with spin welding than with ultrasonic welding.
Additional parts can be entrapped between the upper and lower pieces during
spin welding.

Joining of dissimilar polymers is possible using the spin weld process
although it generally produces lower strength weld joints. By designing the
weld joint with an undercut, the polymer with the lower melting temperature
will flow into the undercut, creating a mechanical union.

Material filler and surface contaminants (e.g. mold release agent) are two
factors that will affect consistency and weld repeatability. Spin welding is
more tolerant of contaminants than ultrasonic welding. Spin welding is also
less affected by hygroscopic polymers, although they may still require
special handling for critical applications. The moisture content can lead to
bubble formation in the joint resulting in decreased weld strength.

Joint Design Considerations

Part joint must be on a circular axis.

Joint design should allow for adequate collapse
distance.

Final orientation of part if necessary.

If at all possible the upper part would be designed
for use with
drive features. Place for upper tool to grasp your part.

The joint area must be designed so that there is no
other part contact interference.

Control Parameters

There are several primary process control parameters that affect weld
quality. They are the surface velocity of the weld joint, press (axial)
speed, weld depth, and hold distance and time.

Surface Speed

For a fixed rotational spin speed (RPM), linear surface speed increases with
weld joint diameter. For a fixed weld joint diameter, surface speed
increases with motor RPM. Smaller diameter parts therefore usually require
more RPM than larger parts of the same material. If the surface speed is too
low, an adequate amount of heat will not be generated to cause sufficient
melting. If the speed is too high, excessive heat in the joint could result
in material degradation or reduction in viscosity leading to material flow
away from the joint.

The selection of the proper surface speed depends to a large degree on
the material and joint geometry of the parts being welded. Some materials,
such as PVC, can be readily welded for a wide range of values, while others
require a narrow range. Commonly quoted values in the literature recommend using +/- 2 m/sec (79 in. /sec.) as an initial testing value. This can be adjusted up or down depending on the results and part configuration.

Press (Axial) Speed

The press speed affects the amount of contact pressure between
the parts being welded, which is required to generate frictional heat. The
larger the speed, the larger the rate of heat rise. In combination with the
surface speed, press speed must be high enough to cause melting at the
interface as opposed to grinding, but not too high as to damage the parts.
Excessive press speed can also lead to stalling of the spin motor as more
torque is required to maintain constant spin speed.

The Dual Servo Spin Welder is capable of operating in two different press
speed modes. With the Constant Torque Option (in SETUP > WELD tab) disabled,
the press speed is constant during the weld. With the Constant Torque Option
enabled, the press speed is variable so as to keep the spin torque constant
(see Chapter 5). The latter case resembles the operation of a pneumatically
driven press, where the press speed is the result of the melt rate under
given air pressure and spin speed conditions.

Selection of the optimum press speed depends on the material and joint
geometry of the parts, as well as the surface speed. A range for initial
experimentation is 0.5 to 2.0 mm/s.

Weld Depth

The determination of the proper weld depth is highly dependent on the
application. The weld joint is typically designed for a specific weld
penetration. Ideally, the weld is sufficiently deep to produce a strong,
hermetically sealed assembly. An excessive depth may lead to the formation
of flash (material that is ejected from the joint area during the weld and
adheres to the assembly), the drawing out of reinforcing filler material and
realignment of the interchain bonds in the weld plane resulting in a weak
axial weld joint, and possibly part distortion.

Since weld depth affects the joint strength and the amount of flash
generated, it is important to design the weld joint properly to meet both
requirements simultaneously. The incorporation of flash trap features is
recommended to produce acceptable appearance without compromising strength.

Hold

During the hold phase, vertical press travel initially brings the molten
parts closer together (dynamic hold) and then allows the molten material to
solidify (static hold). Amourphous plastics will normally take longer to
solidify than semicrystalline plastics. The dynamic hold distance is
typically a small value compared to the weld distance. An approximate
staring point for initial application setup is 10% of weld distance. The
static hold time can vary depending on the size of the part, but is usually
in the 1-3 second range.

Dukane Exclusive Melt Match™

Let Dukane Demonstrate how this Dukane Exclusive
Technology will help you advance your spin welding process.

Constant Torque

"Melt-Match" mode, in which the press vertical speed is
continuously adjusted to match the rate of plastic melt at the
joint. This is achieved by measuring the spin torque and changing
the vertical speed on-the-fly based on this measurement. The
vertical speed is inversely proportional to the spin torque: the
lower the spin torque, the higher the vertical speed, and vice
versa.

The relationship between the spin torque and vertical speed is illustrated
in Figure 5-20. The welder will adjust vertical speed for a measured spin
torque along the lines shown. The Torque Target is the desired spin torque,
which is entered into the Torque (% of max.) field on the screen. The Max
Torque value is 5% larger than the Target Torque. If the measured torque
exceeds the Max Torque, the vertical speed will be 0 until the torque drops
below the maximum. The Max Speed is the maximum allowable vertical speed,
which will occur if the measured torque is 0. This value is entered in the
VERT. Max (mm/s) field on the Weld Parameters screen (in the WELD tab).. The
actual spin torque profile achieved during the weld will depend on the
Torque (% of max.) and the VERT Max (mm/s) settings for a particular
application. For example, if the actual spin torque is consistently below
the specified target, the VERT. Max (mm/s) will need to be increased to
cause the welder to move down faster, causing a rise in the spin torque.